Watermarking is a useful way of conveying hidden auxiliary data with an image. But if the image is distorted in an unknown way (e.g., scaled in size, or angularly rotated), it may be impossible to read the hidden data from the image.
One way to handle this problem is to compare the image with an undistorted version, permitting the distortion to be discerned by pattern matching or related techniques. However, the undistorted image is commonly unknown or not available.
In accordance with a preferred embodiment of the present invention, this problem is overcome by encoding certain marker signals in the image at the time of watermarking. By reference to such signals, retrieval of the auxiliary data is possible even if the image has been scaled or rotated.
In one particular implementation, the marker signals (termed a xe2x80x9csubliminal graticulexe2x80x9d) take the form of an additional signal that is added to the image. This signalxe2x80x94when represented in the spatial frequency domainxe2x80x94appears as a pattern of spots. If the image is scaled or rotated, the pattern changes in known ways. By examining the change in the pattern, the distortion of the image can be inferredxe2x80x94even without access to the original image.